Buggy code:
public class CreditCardValidator extends AbstractValidator < String > { private static final long serialVersionUID = 1L ; public static enum CreditCard { INVALID ( null ) , AMERICAN_EXPRESS ( "American<seq2seq4repair_space>Express" ) , CHINA_UNIONPAY ( "China<seq2seq4repair_space>UnionPay" ) , DINERS_CLUB_CARTE_BLANCHE ( "Diners<seq2seq4repair_space>Club<seq2seq4repair_space>Carte<seq2seq4repair_space>Blanche" ) , DINERS_CLUB_INTERNATIONAL ( "Diners<seq2seq4repair_space>Club<seq2seq4repair_space>International" ) , DINERS_CLUB_US_AND_CANADA ( "Diners<seq2seq4repair_space>Club<seq2seq4repair_space>US<seq2seq4repair_space>&<seq2seq4repair_space>Canada" ) , DISCOVER_CARD ( "Discover<seq2seq4repair_space>Card" ) , JCB ( "JCB" ) , LASER ( "Laser" ) , MAESTRO ( "Maestro" ) , MASTERCARD ( "MasterCard" ) , SOLO ( "Solo" ) , SWITCH ( "Switch" ) , VISA ( "Visa" ) , VISA_ELECTRON ( "Visa<seq2seq4repair_space>Electron" ) ; private final String name ; CreditCard ( String name ) { } } private CreditCardValidator . CreditCard cardId = CreditCardValidator . CreditCard . INVALID ; private boolean failOnUnknown = true ; public CreditCardValidator ( ) { } public CreditCardValidator ( final boolean failOnUnkown ) { } public final CreditCardValidator . CreditCard getCardId ( ) { } protected void setCardId ( final CreditCardValidator . CreditCard cardId ) { } @ Override protected void onValidate ( final IValidatable < String > validatable ) { } protected boolean isLengthAndPrefixCorrect ( String creditCardNumber ) { } public final CreditCardValidator . CreditCard determineCardId ( String creditCardNumber ) { } protected CreditCardValidator . CreditCard isUnknown ( String creditCardNumber ) { } private CreditCardValidator . CreditCard isAmericanExpress ( String creditCardNumber ) { } private CreditCardValidator . CreditCard isChinaUnionPay ( String creditCardNumber ) { } private CreditCardValidator . CreditCard isDinersClubCarteBlanche ( String creditCardNumber ) { } private CreditCardValidator . CreditCard isDinersClubInternational ( String creditCardNumber ) { } private CreditCardValidator . CreditCard isDinersClubUsAndCanada ( String creditCardNumber ) { } private CreditCardValidator . CreditCard isDiscoverCard ( String creditCardNumber ) { } private CreditCardValidator . CreditCard isJCB ( String creditCardNumber ) { } private CreditCardValidator . CreditCard isLaser ( String creditCardNumber ) { } private CreditCardValidator . CreditCard isMaestro ( String creditCardNumber ) { } private CreditCardValidator . CreditCard isSolo ( String creditCardNumber ) { } private CreditCardValidator . CreditCard isSwitch ( String creditCardNumber ) { } private CreditCardValidator . CreditCard isVisa ( String creditCardNumber ) { if ( ( ( creditCardNumber . length ( ) ) == 13 ) || ( ( creditCardNumber . length ( ) ) == 16 ) ) { if ( creditCardNumber . startsWith ( "4" ) ) { <START_BUG> return CreditCardValidator . CreditCard . SWITCH ; <END_BUG> } } return CreditCardValidator . CreditCard . INVALID ; } private CreditCardValidator . CreditCard isVisaElectron ( String creditCardNumber ) { } private CreditCardValidator . CreditCard isMastercard ( String creditCardNumber ) { } protected final boolean isChecksumCorrect ( final String creditCardNumber ) { } }

Fix:
return CreditCardValidator . CreditCard . VISA ;